Reflector lets LEDs replace light bulbs

Portland, Ore.  Light bulbs create more heat than light. But a professor at Rensselaer Polytechnic Institute (Troy, N.Y.) has invented a 99 percent efficient, omnidirectional reflector that he claims will allow light-emitting diodes to replace conventional light bulbs within five years, saving energy and related cost.

Fred Schubert, Wellfleet senior constellation professor of the Future Chips Constellation at Rensselaer, has demonstrated his LEDs in red and blue but is pursuing white LEDs for light bulb replacement.

The National Science Foundation recently granted Schubert $210,000 to create a commercial version of his patented omnidirectional reflector within three years. The funding comes on top of a $250,000 Defense Advanced Research Projects Agency grant that enabled Schubert to develop the current prototype over the past two years.

"In an LED, light emits from inside the semiconductor in every direction, but our mirror reflects light equally well no matter what the angle of incidence. Other types of reflectors are only efficient when the angle is normal [90° perpendicular to the surface]," Schubert said.

The omnidirectional reflector is made by integrating a silver mirror onto the same chip as the LED. In more detail, a layer of dielectric is lithographically patterned with arrays of micron-sized holes and is deposited on the bottom of the LED semiconductor layer. Then the mirrored reflector is deposited as a thin film of silver on the dielectric. Where it penetrates through the patterned holes in the dielectric, it makes arrays of microcontacts with the semiconductor beneath.

Current then flows through the backside silver microcontacts to activate the semiconductor layer, causing it to emit omnidirectional light that is reflected off the silver back and through the top of the LED at more than 99 percent efficiency.

"The LED market has already been growing about 10 percent a year. With our new reflector becoming adopted, we think the market will continue growing at 10 percent or more per year for the foreseeable future," said Schubert.

Schubert predicts that the white LEDs developed under the NSF contract will be capable of lighting homes, businesses, public buildings (including demanding indoor environments such as museums) and even outdoor environments. Twenty-five percent of today's electrical-energy consumption in the United States is for lighting, so white LEDs could be a major energy saver, emitting more light per dollar and generating less heat, he said.

Also, the "tunability" of white LEDs will enable smart lighting systems, Schubert said. "For instance, I can envision lighting systems that change color slightly during the course of the day to keep our circadian clocks in rhythm with what time it is. You can't economically build such a system with light bulbs."